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202               PRACTICAL ORGANIC CHEMISTRY

tube furnace to 140—150° for eight to nine hours. After cooling,
the capillary is opened and the tube removed from the furnace.
The bromine will have completely disappeared, and colourless
crystals of bromobenzoic acid now fill the tube. The contents
are removed, filtered, and boiled with water (100 c.c.) in a
basin to drive off unchanged benzoic acid. The liquid is
cooled, filtered, and the bromobenzoic acid crystallised from
hot water. Yield, 5 grams.

CGH5CO.OH + Br2 = C6H4Br.CO.OH + HBr.
Properties.—Colourless needles ; m. p. 155°,

PREPARATION 93.
CCH5.CHOH
Benzoin,         |
C0H6.CO
1                            Liebig, Wohler, Annalen, 1832, 3, 276; Zinin, Annalen, 1840,
34, 186.
25 grms. benzaldehyde.
5     „     potassium cyanide (in 20 c.c. water).
50   c.c.   absolute alcohol.
,]                              The mixture of benzaldehyde, potassium cyanide and alcohol
\                           is heated on the water-bath  with an  upright  condenser for
Jl                           about half an hour.    On cooling the liquid, the benzoin separates
1  ,                          out as a mass of small colourless crystals, which are filtered and
'j                           washed with a little alcohol.    Yield, about 20 grams.    A portion
;,:                          of the substance may be purified by recrystallisation from spirit.
|                                              2C6H5COH = C6H5CO.CH(OH).CGH5.
|                             Properties.—Colourless prisms; m. p. 137°;  slightly soluble
*                  -        in water ; soluble in alcohol and ether.
\                             Reaction.—Add Fehling's solution to benzoin dissolved in
,|                          alcohol. Benzil is formed and cuprous oxide precipitated,
•vj                          Benzil is also formed on oxidation with nitric acid.
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